Cross Currency Receipt

We are working on requirement with in AR, The transaction is in EUR currency and the receipt is in ILS. As per current design, when business tries to apply a transaction by default invoice amount in EUR currency will be present on the Applied Amount filed . At this stage business only knows the Allocated receipt amount in ILS and user has to back calculate the converted applied amount in EUR to input/update the filed. so that system calculate the cross currency rate. alternatively enter the converted applied amount and cross currency values so that system calculate the allocated receipt